1. Field of the Invention
This invention relates to the communication of signals from within a cased borehole or other metallic conduit and, more particularly, to a wireless communication system which utilizes current generated within a short segment of an electrically conductive conduit to develop electromagnetic energy for communicating a signal, generated by a transmitter located within the conduit, to a remote receiver.
2. Description of the Prior Art
One of the present methods to improve oil and gas flow in oil wells is to inject acid or mixtures of water and sand at high pressures into the producing formation strata in the oil well. This process is commonly referred to as a well stimulation process.
In order to design and operate a successful well stimulation process, it is important to determine a number of down-hole conditions. Of these conditions, the most important are the actual bottom-hole pressure and temperature measured at the face of the producing formation while the stimulation process is being performed; i.e., the "real-time" bottom-hole pressures and temperatures. If those "real-time" parameters were available for evaluation during the stimulation operation, then the stimulation process is improved, and overall stimulation costs are reduced.
Among the existing methods of obtaining data relating to the down-hole pressures and temperatures during well stimulation procedures are the following:
1. Data can be obtained using a measuring instrument recorder which is disposed at the bottom of the hole and is then retrieved after the stimulation process is completed. Unfortunately in using this technique, the down-hole conditions can only be replayed at the end of the stimulation process and this data is not "real-time" data.
2. Bottom-hole conditions can be calculated based on conditions measured at the surface that estimate the wellbore conditions. However, the accuracy of these indirect measurements is generally poor because the measured and estimated conditions are constantly changing throughout the stimulation process.
3. Sensing devices can be placed down-hole with an electrical cable or wireline communicating between the sensing device and the surface. This method can provide a reliable communications link but is costly, and the cable or wireline is prone to tangling, breaking or interfering with the fluid flow in the borehole.
In addition, a number of other prior art wireless wellbore communication systems are known. Many of these systems are designed specifically to be used in the drilling industry as "measurement-while-drilling" systems. Typically these systems use apparatus mounted directly above the drill bit to record the drilling conditions in the vicinity of the drilling bit. The drilling data is modulated into an electric signal and transmitted by propagating electromagnetic energy through the strata adjacent to the drill pipe and decoding those signals at the surface. From these signals the conditions of the drilling environment and adjacent strata can be determined. Examples of such technology can be seen in U.S. Pat. Nos. 4,578,675 and 4,739,325 issued to MacLeod. The MacLeod devices include instrumentation that produces and receives signals at the bottom of the well hole. However, the MacLeod device is not readily adaptable for use in pre-drilled holes cased with an electrically conductive conduit. Also, the MacLeod device cannot be used with the well stimulation procedures because such procedures are employed after the casing is installed in the well hole.
U.S. Pat. No. 3,831,138 issued to Rammner discloses a method of communicating drilling conditions from a position near the drill bit to the surface using electric signals. This device operates by creating a dipole in the body of the drill tube just above the drill bit. The dipole transfers electric current to the strata in the vicinity of the drill bit, and this current is propagated through the strata to the surface in the form of a current field. The Rammner device cannot be utilized where there is a conductive casing in the borehole, such as a well casing,
Yet another method of communicating with the surface is shown in U.S. Pat. No. 4,839,644 issued to Safinya et al., which discloses a system for wireless, two-way electromagnetic communication along a cased borehole which has a metallic tubing string extended down into it. One part of the communications system is located at or near the base of the tubing, and another part is located at the surface. Communication is achieved by transmitting electromagnetic energy to the surface through the casing/tubing annulus. A disadvantage of this system is that effective operation requires the tubing to be insulated from the casing, in order to eliminate electrical shorts caused by the tubing-casing contact. Thus, non-conductive spacers and a non-conductive fluid must be provided in the annulus space between the tubing and the casing, thereby increasing the cost, making the Safinya device logistically difficult to employ, and commercially inapplicable in most well stimulation operations.
Yet another wireless communication system is disclosed in U.S. Pat. No. 3,967,201 issued to Rorden. This patent discloses a method of communication whereby low frequency electromagnetic energy is transmitted through the earth between two generally vertically orientated magnetic dipole antennae. One antennae, located at a relatively shallow depth within the borehole, includes an elongated electrical solenoid with a ferro-magnetic core and generates relatively low frequency electromagnetic energy which propagates through the earth. The device can be used in a cased borehole; however, as admitted in the specification (col. 3, lines 17-19), communication is much more difficult if the casing is present in the borehole. Also, the specification describes art for communicating at shallow depths (0-2000') and for controlling the operation of a shallow down-hole valve and does not disclose how this technology can be used for communication of information from much deeper holes and through the relatively hostile environment created by well stimulation techniques.
Notwithstanding all the above described prior art, the need still exists for a relatively inexpensive, routinely usable, efficient method of wireless communication from the bottom of an encased borehole to the ground surface.